(a) Field of the Invention
The invention concerns a process which is rapid, and economical to produce collector-electrode assemblies consisting of a metallized insulating plastic support on which there is first provided a controlled electrochemical deposit of a metal which is sufficiently conductive, which is thereafter coated with a film of a material of an electrode (anode or cathode) of a thin-film cell. The electrochemically deposited metal is selected for its compatibility with the material of said electrode. This process enables to prepare collector-electrode assemblies which are particularly well adapted for thin film electrochemical cell utilizing polymer electrolytes. The embodiments of the invention enable for example to optimize the characteristics of these collector-electrode assemblies as well as the performances and the designs of the cell. The main advantages of the processes and devices of the invention concern particularly the optimization of the thickness and of the conductivity of the conductive coating, the ease of handling in machines, of the assemblies supported on plastic film and the choice of certain designs which are particularly advantageous for the use of thin film cell. These advantageous designs are made possible, for example, because of the control of the surfaces and the shapes of the conductive coatings on which the materials of the electrode are applied.
(b) Description of Prior Art
A collector is defined as presenting an additional metal which is applied on the electrode to drain the electrons from the cell without an appreciable ohmic drop.
The recent development of polymer electrolyte batteries, for example those based on lithium, present substantial possible advantages on the point of view of electrochemical performances and techniques of application. These cell, which are entirely in solid state, enable in principle a substantial range of application which include electronic micro-cell up to the large generators intended for electrical vehicles. However, important technological difficulties must be overcome in order to be able to optimize the performances and to develop economical processes of manufacture of this new type of cell.
The polymer electrolytes are, indeed, not very conductive with respect to the organic liquid electrolytes presently used for primary or rechargeable lithium cell. The consequence is that electrochemical cells must be prepared from very thin films (total thickness of 5 to 200 microns). Important film surfaces should therefore be used to compensate for the low conductivity of the polymers and to give the desired powers and energies during storage.
In previous works, the Applicant has shown how to prepare a cell and its electrochemical components in the form of thin films (U.S. Pat. Nos. 4,517,265; 4,851,307; 4,758,483; 4,824,746; French Patent 87 08539 and U.S. Pat. No. 4,897,917). However, when the thickness of the active electrochemical components of the cell is reduced (anode, electrolyte and cathode), it is important to reduce, in the same proportion, the passive Components (collectors and electric insulating materials). Otherwise, what will be gained, in power or in electrochemical performance, by increasing the surface of the cell will be compromised by the loss of massic or volumic energy content, as a result of the proportion taken by the passive components. Moreover, if the Applicant has shown how the components of the cell may be prepared rapidly and at low cost, it is not the same with the collectors (and to a lesser degree of the insulating materials) presently available, of which the cost become too high when the required thicknesses are of the order of 10 microns to a few tenths of micron.
Among the processes presently available to manufacture thin collectors of the order of a few microns, lamination is more current. However, few metals are sufficiently malleable to be laminated at these thicknesses (Au, Sn, Al for example) and, generally, the costs increase rapidly for thicknesses below 50 to 5 microns, by virtue of the number of passes required for lamination, losses of materials, as well as the problems of hardenings and contamination by lubricants. Moreover, the commercially available thicknesses (12 microns for stainless steel, 6-7 microns for aluminum) are still too elevated and penalizing in terms of cost, material and dead weight or volume, for most of the applications intended for polymer electrolyte batteries.
Electro-forming is a technique of plating where the free and self-supported metallic film is obtained by electrochemical deposit, from an electrolytic solution, on an electronic conductive mandrel. The costs of the manufactured products stand at about US$ 1/pi.sup.2 and the minimum thicknesses which are available are 6 microns nickel and 10 microns for copper. This technique is applicable to certain materials only and, the necessity to peel and handle a free film after the deposit requires certain materials only and, the necessity to peel and handle a free film after the deposit requires minimum thicknesses otherwise all mechanical behavior is lost. These films are currently used as weldable conductive sheets in the manufacture of printed circuits.
Table I illustrates the importance that the weight and thickness of the current collectors may take for thin film polymer electrolyte cell. A comparison is made, by way of example, on the basis of a battery of type AA in which the useful volume is fixed at 6.6 cm.sup.3. The battery is made of the following elements in the form of films co-wound into a spiral: Li/electrolyte polymer/TiS.sub.2 /collector (Al or Ni)/polypropylene insulating film (PP) as illustrated in FIG. 3. This table is based on passive components (collector insulating films) selected from products which are presently available in the trade (Al 12.mu., Ni 7.mu., PP 5.mu.) and on two formulations of polymer electrolyte batteries which are typical in the technology which is presently developed, wherein the total thickness of the electrochemical components, Li/polymer electrolyte/TiS.sub.2 is respectively fixed at 56 and 180.mu..
TABLE 1 ______________________________________ Thicknesses of electrochemical components Li/polymer electrolyte/TiS.sub.2 56 microns total 180 microns total Thicknesses of passive components Al 12.mu./ Ni 7.mu./ Al 12.mu./ Ni 7.mu./ PP 5.mu. PP 5.mu. PP 5.mu. PP 5.mu. ______________________________________ Wh/1 220 236 260 267 Wh/Kg 140 111 160 160 ______________________________________
It will be noted that in this table, the passive components (collectors/insulating materials) presently available become very penalizing for the electrochemical characteristics of a cell (Wh/l, Wh/Kg) with polymer electrolyte and the latter is prepared in thin form, i.e. when the total thickness of the active chemical components decreases. These penalties result from the excess of volume and weight of the conductive metal present therein. These values are given to assist in characterizing the prior art only; the thicknesses of 56 to 180 microns are selected arbitrarily. For batteries thicknesses lower than these values, still more important penalties are to be expected. Moreover, by way of illustration, the cost of an electro-formed sheet of nickel, 1 $/pi.sup.2, used in the cells of table I would represent by itself only from $0.40 to $1.10 per battery AA. Such a cost is unacceptable commercially for this type of readily available battery. The cost of aluminum under these conditions is less, but this metal is penalizing on the point of view of volumic energy and limits the designs of the proposed cell because it is unstable in the presence of lithium and its alloys.
It is known, on the other hand, that these metallized dielectric plastic films may be used as electrodes in electrostatic condensers, see for example, European Patent No. 0 073 555 and French Patent No. 2,637,118. The metals generally deposited on plastic are in this case aluminum, zinc and their alloys. These metallizations are generally obtained, under vacuum, by thermic evaporation or by other assisted processes of evaporation: cathodic projection or electron beam. The thickness thus obtained are however very low, typically 100-500 .ANG. and the surface resistances are consequently very high, 1-100 .OMEGA./square. In addition to the fact that the metals which are known and deposited are not chemically stable with the anode of polymer electrolyte cell, the surface conductivities thus obtained are not sufficient to permit the draining of the ranges of currents provided for the cell of average or large sizes. The processes of metallization under vacuum are known to be limited to thicknesses lower than about 1000 .ANG.(0.1.mu.). At these thicknesses, the process of metallization becomes slow and costly and the deposits obtained are of less quality. These electrodes of electrostatic condensers are therefore not applicable as collector in most of the polymer electrolyte lithium cell, except possibly in the case of the metallization of aluminum applied to a positive electrode in low size batteries, operating at 25.degree. C. and where the mean current densities are low and the lateral distances to be collected are low (see equation 1, hereinafter).
There is therefore a range of critical thicknesses located between 5 and 0,1 microns for which, presently, there is practically no adequate collectors for the technology of polymer electrolyte lithium batteries. The present invention concerns current collectors in which the possible range of thicknesses corresponds to this specific need.